1. Field of the Invention
The present invention relates to a semiconductor device having a buried insulated gate and a method of manufacturing the same and, more specifically, to a vertical power MOSFET (metal oxide semiconductor field effect transistor) having a U-shaped trench gate to enhance the breakdown voltage of a gate insulation film between a gate and a source.
2. Description of the Related Art
FIGS. 1A to 1C schematically show the constitution of a conventional vertical power MOSFET having a U-shaped trench gate. Of these figures, FIG. 1A is a plan view of the vertical power MOSFET, FIG. 1B is a cross-sectional view taken along the line IB--IB of FIG. 1A, and FIG. 1C is a cross-sectional view taken along the line IC--IC of FIG. 1A.
As shown in FIGS. 1A to 1C, a P-type base layer 2 is formed on the surface of an N-type semiconductor substrate 1, and an N.sup.+ -type emitter layer 3 is formed on the layer 2. A plurality of trenches 4a serving to constitute an element operating region (MOS region), are provided in parallel with one another to such a depth as to reach the semiconductor substrate 1 through the base and emitter layers 2 and 3. To connect the trenches 4a one another, a trench 4b serving as a gate wiring region, is formed to cross the trenches 4b at right angles.
Polysilicon serving as a gate 8 is buried in each of the trenches 4a and 4b, while a gate insulation film including an oxide film 5, a nitride film 6 and an oxide film 7 is interposed between the gate 8 and the surfaces of the trenches. A gate electrode wiring layer 9, which is formed from part of the polysilicon buried in the trenches 4a and 4b, extend along the trench 4b.
In the conventional vertical power MOSFET having the above constitution, its surface excluding the gate electrode wiring layer 9 is coated with a protection film (not shown), and a drain electrode D, a source electrode S and a gate electrode G are drawn from the semiconductor substrate 1, N.sup.+ -type emitter layer 3 and gate electrode wiring layer 9, respectively.
A commonly-known vertical power MOSFET can be operated as a high power device capable of dealing with a large amount of current if the number of gates increases as the number of trenches does.
In the conventional vertical power MOSFET, however, the gate electrode wiring layer 9 covers corner portions of the N.sup.+ -type emitter layer 3. For this reason, an electric field concentrates upon the corner portions, and the breakdown voltage of the gate insulation film directly under the corner portions drops. Thus, the breakdown voltage of the gate insulation film easily decreases between a gate G and a source S. Though the breakdown voltage can be improved by increasing the thickness thereof, whereas the thickness cannot be increased more than required since the operating characteristics of the device depends upon the thickness of each of the films 5 and 6.